Hydrogen sulfide is a toxic and corrosive gas. It can diffuse into drilling fluid from earth formations during the drilling of boreholes such as for oil and gas wells. Hydrogen sulfide that is present in a high enough concentration may require removal and special handling. Downhole concentrations of hydrogen sulfide can vary widely. Hence, it would be important to determine the concentration of hydrogen sulfide downhole in order to efficiently plan for well production. It would be well received in the drilling and geophysical exploration industries if a downhole sensor for detecting and measuring a concentration of hydrogen sulfide could be developed.